1. Field of the Invention
This invention relates to a serial recording apparatus for forming dot images on a sheet on the basis of print data while scanning in the direction of a print column by a recording head carried on a carriage.
2. Related Background Art
As a recording apparatus for a printer, a facsimile apparatus or the like, use has widely been made of a recording apparatus of the type in which a recording head is carried on a carriage movable in the direction of the print column and the recording head is driven on the basis of print data while scanning by the recording head, thereby forming dot images on a sheet, i.e., a serial recording apparatus.
FIGS. 12 to 15 of the accompanying drawings are fragmentary perspective views showing the essential portions of the various types of the driving mechanism for said carriage.
FIG. 12 shows a carriage driving mechanism of the rack-and-pinion type.
In FIG. 12, a carriage 62 carrying a recording head 61 thereon is supported for movement along a guide shaft 63 and a guide rail 64. A pinion 65 rotated by a carriage motor (not shown) is supported on the carriage 62 and is in meshing engagement with an elongated rack 66 installed on the basis of a recording apparatus, and the position and movement of the carriage 62 can be controlled by controlling the rotation of the pinion 65 by the carriage motor. A rotary encoder 67 for detecting the rotation of the pinion 65 is mounted on the carriage 62.
FIG. 13 shows a carriage driving mechanism of the belt transmission type.
In FIG. 13, a carriage 72 carrying a recording head 71 thereon is connected to a belt 75 passed over a pair of pulleys 73 and 74. One pulley 73 is a driving pulley rotatively driven by a carriage motor 76, and the other pulley 74 is a follower pulley.
Accordingly, the movement and position of the carriage 72 can be controlled by controlling the revolution of the motor 76.
FIG. 14 shows a carriage driving mechanism of the wire rope type.
In FIG. 14, a carriage 82 carrying a recording head 81 thereon is driven by a carriage motor 83 through a wire rope 84.
The opposite ends of the wire rope 84 are connected to the carriage 82, and the wire rope 84 is passed over four guide pulleys 85A, 85B, 85C and 85D and the movement and position thereof are controlled by a driving pulley 86 rotated by the motor 83.
FIG. 15 shows a carriage driving mechanism of the lead screw type.
In FIG. 15, a carriage 92 carrying a recording head 91 thereon is threadably engaged with a threaded bar 93, which is rotatively driven by a carriage motor 94 through gears 95 and 96. The direction of movement and the speed of movement of the carriage 92 are controlled by the direction of rotation and the speed of rotation of the threaded bar 93.
On the other hand, as a speed control system for keeping the speed of movement of the carriage constant, use has been made of an open loop system using a pulse motor, or a closed loop system in which the driving voltage of a DC motor or the oscillation frequency of a pulse motor is controlled in confirmity with the output of the encoder 67 as shown in FIG. 12.
Also, as regards the printing system in the recording head, the wire dot system, the heat transfer system or the piezo ink jet system is the mainstream, and the response frequency of each element (dot forming element) of the recording head is 1000-3000 Hz in the wire dot system, 500-1500 Hz in the heat transfer system, and 1000-3000 Hz in the piezo ink jet system, and further, the dot density in the image output by these systems is in the range of 7 dots/mm to 14 dots/mm.
However, in the prior-art serial recording apparatus, the rotational movement of the motor for driving the carriage has been converted into rectilinear reciprocal movement through a rack and a pinion, pulleys and a belt, a wire rope or a lead screw as shown in FIGS. 12-15, respectively, and this has lead to the necessity of a mechanism portion for transmitting and converting the power. To maintain dot position accuracy in the dot density area as previously mentioned, it has become necessary to increase the frequency of the pulse motor or to make the pitch of the encoder (such as the encoder 67 of FIG. 12) fine and therefore, the carriage driving mechanism has been complicated in structure and it has been difficult to make it compact.
There has also been the problem that due to the back-lash between the elements in the mechanism portion, the back-lash of the guide portion for rectilinear guide and further the back-lash of meshing portions such as gears, the noise of the carriage during the reciprocal driving thereof becomes so great that it is difficult to make the noise low.
Also, the presence of back-lash in the mechanism portion has made it difficult to improve dot position accuracy.
In the wire dot system, the heat transfer system and the piezo ink jet system which are the conventional printing systems, it is necessary from the limitations in the recording system to keep the printing period (the driving period of the recording head) constant and therefore, design is made such that the speed of movement of the carriage to be synchronized with the printing period is also kept always constant.
The control for making the speed of the carriage constant has been executed by a method using a motor having a sufficient output torque in reserve in a case where the carriage driving motor is a pulse motor and open loop control is effected, and further has been executed also by a method of effecting speed control by a closed loop system of a DC motor or a pulse motor and an encoder.
Here, description will be made of the relation between the printing operation and the speed of movement of the carriage in the various prior-art printing systems.
FIGS. 16A to 16C of the accompanying drawings is a graph illustrating the timing of the printing operation of the wire dot system. FIG. 16A shows the repeated printing period of the print wire, FIG. 16B shows the time for which electric power is supplied to the magnet coil of each print wire of the wire dot head, and FIG. 16C shows each flight cycle until the print wire begins to move and prints and returns.
FIG. 17 of the accompanying drawings is a schematic cross-sectional view of the wire dot head.
In the case of the wire dot system, when the head response frequency is 2500 Hz approximate to the highest speed, the repeated printing period of the same print wire 52 is 400 .mu.s {FIG. 16A} as shown in FIGS. 16 and 17, and usually the time for which electric power is supplied to the magnet coil 51 is set to the order of 200 .mu.s {FIG. 16B}.
On the other hand, about 390 .mu.s {FIG. 16C} is necessary as the shortest flight time from after the print wire 52 begins to move until it impacts the surface of a sheet (a recording medium such as printing paper) 53 and returns and therefore, under the condition approximate to the above-mentioned highest speed, the stable operation of the print wire 52 will be moved unless the fluctuation of the printing period is controlled to the order of 10 .mu.s (400 .mu.s-390 .mu.s).
Also in the case of the piezo ink jet system, as in the case of the above-described wire dot system, the fluctuation of the speed of the carriage is limited by the time for the return of a piezo vibration plate and the return of the meniscus in the orifice, instead of the wire flight time.
Further, in the case of the heat transfer system, it is very difficult to realize a printing period of frequency 2500 Hz and comparison at the same level is difficult, but a longer time for power supply is required as compared with the aforedescribed two systems and therefore, greater stability of the carriage speed becomes necessary.
FIG. 18 of the accompanying drawings diagrammatically illustrates the control system for the carriage driving system in the prior-art serial recording apparatus.
In FIG. 18, in the control circuit (MPU) 101 of the recording apparatus, there are provided a ROM 102 storing a control program, etc. therein and a RAM 103 including a working area such as a buffer register temporarily storing various data therein, and various data from a host apparatus are sent to the control circuit 101 through an interface (I/F).
The control circuit 101 controls a recording head 105 through a head drive circuit 104 and also controls a carriage motor 108 through a motor timing control circuit 106 and a motor drive circuit 107, and further controls a sheet feed motor 110 through a sheet feed motor drive circuit 109.
On the other hand, the output signal of an encoder for detecting the position and speed of the carriage is made by a photosensor 111, is shaped into a pulse wave form by a wave form shaping circuit 112, and is transmitted to the head drive circuit 104 and the motor timing control circuit 106, whereby the synchronizing control of the scanning of the carriage and the printing operation of the head is effected.
As is apparent from the foregoing description, the carriage driving control system in the prior-art serial recording apparatus is of complicated construction.
FIG. 19 of the accompanying drawings is a fragmentary perspective view showing the construction of the carriage of a wire dot recording apparatus.
In FIG. 19, a recording sheet 121 as a recording medium such as printing paper or a plastic sheet is held in intimate contact with the surface of a platen 122 which serves also as a sheet feed roller, and a carriage 125 is movably supported by a guide shaft 123 and a guide rail 124 installed forwardly of and parallel to the platen.
A wire dot head 126 containing therein a plurality of (e.g. 64) print wires and drive means therefor, and an ink ribbon cassette 127 for supplying an ink ribbon for transfer are mounted on the carriage 125.
FIG. 20 of the accompanying drawings is a fragmentary perspective view showing the construction of the carriage of a heat transfer recording apparatus.
In FIG. 20, forwardly of a platen 132 for backing up a recording sheet 131, guide shafts 133 and 134 are installed parallel thereto, and a carriage 135 is movably supported by these guide shafts 133 and 134.
On the carriage 135, a thermal head 136 having a plurality of (e.g. 64) heat generating elements is supported for movement up and down, and an ink ribbon cassette 137 for supplying an ink ribbon for transfer between the thermal head 136 and the recording sheet 131 is further mounted.
As is apparent from FIGS. 19 and 20, in the construction of the carriage of the recording apparatus of the wire dot system or the heat transfer system, the load with which the ribbon is taken up and the load with which the recording head 126, 136 contacts with the sheet 121, 131 and the ribbon are added as the load fluctuation elements during the movement of the carriage 125, 135, and this has also led to the problem that the carriage drive motor and the driving circuit therefor become bulky and complex.
As described above, in the prior-art serial recording apparatus, even if various carriage driving methods and each printing method are combined skillfully, an attempt to execute highly minute printing at a high speed would lead to complex and bulky structure as well as to great energy of operation sound, and it has been very difficult or impossible to make the apparatus compact and light in weight and reduce the noise.
In order to solve such problems, another recording apparatus in which a recording head is carried on a carriage movable in the direction of a print column, recording is effected on a sheet by the recording head and which uses an ultrasonic motor as a motor for driving the carriage is known from Japanese Patent Application Laid-Open No. 62-77968 and Japanese Patent Application Laid-Open No. 62-77969.
In the driving of this conventional carriage using an ultrasonic motor, the construction has been simplified and the noise has been reduced, but no sufficient improvement has been made in making the apparatus compact and reducing the cost of the apparatus. In the drive system for the conventional carriage using an ultrasonic motor, the guide rail of the carriage is made into an endless annular shape and is used as the vibration plate of the ultrasonic motor and a surface wave is generated in this endless annular guide rail by two piezo motors of different phases and is used as a travelling wave to drive the carriage. The necessity of such endless annular guide rail leads to the formation of annular portions at the opposite ends of the guide rail, and such annular portions at the opposite ends have made the entire apparatus very bulky and costly. This has proved a great hindrance in instrumenting the carriage drive using an ultrasonic motor.
Also, U.S. Pat. No. 4,672,256 discloses an ultrasonic motor for linear driving, but even if this ultrasonic motor is used in a printer to drive the carriage, an endless annular vibration plate serving also as a guide rail or parallel to a guide rail is required and therefore, the entire apparatus has become very bulky and costly, and this has proved a hindrance in providing the apparatus as a product.